belt sander repair, suppression capacitor- ELECTRONICS EXPERTS

[hmm]

My belt sander gave out today.

Having fixed my washing machine over and over by buying an 8p resistor, I opened up the sander, thought I'd have a go.

First to check was the switch and power in.

I found that the power in does not go from switch to motor, but via a supression capacitor (X2 suppression cap,0.22uF 275Vac 22.5mm) which is in parallel.

1) Can anyone tell me how to test a capacitor (I only have a multimeter)

2) when I unplug the sander and accidenlty touch the three pins of the plug, I get a shock (does that mean the capacitor is stuffed and is throwing some current back out?)

3) The sander does not work without the capacitor. (I do not know what a supression capacitor does, therefore do not know if the sander should work without it)

4) So, do you think it is the 25p capacitor (£7 with postage - rs components) or the sander?

Thanks

 

[Talbot]

worth just getting one and trying it at that price.

 

[VicS]

[ QUOTE ]
I found that the power in does not go from switch to motor, but via a supression capacitor (X2 suppression cap,0.22uF 275Vac 22.5mm) which is in parallel.

[/ QUOTE ]
Sorry but that statement does not make sense in as much that if the power goes via a capacitator it will be in series not in parallel.

If you have a double capacitator then you should find that one half is wired between live and earth and the other between neutral and earth ie in parallel. If it is like that then you can remove it altogther. Its purpose is to supress interferencce on radios and TVs. If the unit does not work withouout it I suspect it is knackered. Worth taking a look at the brushes though.

In other electric motors, which you will recognise by not having brushes ie induction motors a capacitator is used to provide two "phases" as such motors will not run, or more acurately, start on one phase. (Oversimplified I'm afraid)

A capacitator will show a complete open circuit when tested with a meter on the resistance ranges. More sophisticated meters do sometimes have a capcitance range though.

I had trouble with a very old electric drill when its suppression capacitator develoled a partial short. It blew the fuse. Eventually I put a larger fuse in the plug and then the capacitator blew itself apart. I removed the remains and still use the drill.

 

[hmm]

Thanks,

Yeah, I know what you mean - I figure a cap should be in series (if it is a supression cap) - (I've only been told it is a supression cap) - my first thoughts were that it was powering the motor somehow, but I have brushes, so I guess not.

The switch is not what I'm used to, so I could have described it wrongly, but listen to this:

First, the switch looks like this:
open closed
A / B A--B
C / D C--D

A is the blue (neutral (earth)) input
C is brown live input

when the switch is closed, A shows continuity to B
and C shows continuity to D as in 'diagram' above.

Now the capacitor is wired across D and B, and also coming from D and B are two brown wires disappearing into the motor.

Do you see why I said parallel? Was I wrong?

Thanks

 

[hmm]

Even better if I can get one for 25p from a maplins shop over the counter (no delivery)
and I've just found that Maplin do have 'em - probably not in shop though.

It was a very cheap sander (probably why it bust!)

but I'll still have a go- it's a way to learn.

 

[john_morris_uk]

The capacitor is a noise/interference supression capacitor and is in parallel as you have correctly noted. If it failed, it would either fail 'short' and the fuse would blow, or it would fail open, and the sander would still work.

If you want my money on what has gone wrong:

The most likely cause of the sander failing is:

1. A break in the power cable. (Possible but not altogethr likely)

2. Worn out brushes in the motor. (Quite likely.)

3. Failure in the windings/armature of the motor. (possible - but not as likely as 2.)

4. Failure of the switch. (just as likely as 2.)

A multi-meter on 'resistance' will check continuity of the cables, the switch and even whether the brushes as connecting. A bit of logic in the approach will soon tell you which bit has failed.

Ask again if you want more advice. Two minutes on the bench should tell what has gone wrong.

Regards

John

 

[William_H]

In some admittedly rare cases a suppression capacitor is in the form of a feed through capacitor. There is a rod through the middle which carries the main current and the capacitor is built around the outside of this rod. The reason for this is that a normal capacitor can exhibit inductance (significant at higher frequencies) in the leads that connject to the interfering circuit. The feed through capacitor minimises that inductance. So to test this type simply check the continuity from one end to the other. NB for the capacitor to work the metal body must be earthed. The body being the other end of the capacitor.
Many devices have an interference suppressor which has either one or both power leads feeding into it and then out again. The suppressor consists of an inductance (coil) in line with the power and capacitor from each power lead to ground. Possibly a capacitor at both input and output to ground.

So as you have an armature and brushes type motor it is unlikely you have a phasing capacitor where the power goes through the capacitance itself. The fact that you got a slight shock from the capcitance holding power indicates that the capacitor has capacitance.
They can be tested by puting an ohm meter across the capacitor you usually note that resistance is initiallly low then rises to high. It should be very high after a few seconds. Check again by reversing the meter leads and you should find again low resistance as the meter discharges then charges the C in the opposite direction. The time it takes to charge the C depends on the capacitance value and is not normally noticeable unless the C is more than a few microfarrads.(ufd) (A device can be a capacitor with a value between 1 farrad and 1 picofarrad a trillion to one range.)

Any way lesson over. It is unlikely the C is giving the failure. Probably cord, brushes or windings in that order.
It is common for the 2 power wires to go through 2 field windings then to the 2 brushes. So if you are VERY CAREFULL you should be able to measure 240 volts at the brush holders one to the other with power connected. Presence of this voltage may indicate armature or brushes faulty. This voltage will usually be a lot less when it is running due to volt drop in the field windings. It is much safer to check with multimeter on ohms however. Brushes should have a noticeable spring pressure onto the commutator and be like nearly 1cm long. If they get too short with wear they don't have enough spring pressure. They can also stick in the holder. good luck ..olewilll

 

[hmm]

Thanks John and William and all

It was the brushes, had some new ones - job done.

Thank goodness for multimeters.

Cheers